The field of the disclosure relates generally to fiber communication networks, and more particularly, to optical networks utilizing simultaneous upstream communications.
Telecommunication networks include an access network through which end device subscribers connect to a service provider. Bandwidth requirements for delivering high-speed data and video services through the access network are rapidly increasing to meet growing consumer demands. At present, data delivery over the access network is growing by gigabits (Gb)/second for residential subscribers, and by multi-Gb/s for business subscribers. Present access networks are based on passive optical network (PON) access technologies, which have become the dominant system architecture to meet the growing high capacity demand from end devices.
Gigabit PON and Ethernet PON (EPON) architectures presently provide about 2.5 Gb/s data rates for downstream transmission and 1.25 Gb/s for upstream transmission (half of the downstream rate). 10 Gb/s PONs (XG-PON or IEEE 10G-EPON) have begun to be implemented for high-bandwidth applications, and a 40 Gb/s PON scheme, which is based on time and wavelength division multiplexing (TWDM and WDM) has recently been standardized. A growing need therefore exists to develop higher/faster data rates per-subscriber to meet future bandwidth demand, and also increase the coverage for services and applications, but while also minimizing the capital and operational expenditures necessary to deliver higher capacity and performance access networks.
One known solution to increase the capacity of a PON is the use of WDM technology to send a dedicated wavelength signal to end devices. Current detection scheme WDM technology, however, is limited by its low receiver sensitivity when coherent signals are employed, and also by the few options available to upgrade and scale the technology, particularly with regard to use in conjunction with the lower-quality legacy fiber environment. The legacy fiber environment requires operators to squeeze more capacity out of the existing fiber infrastructure to avoid costs associated with having to retrench new fiber installment. Conventional cable access networks typically include six fibers per node, servicing as many as 500 end devices, such as home subscribers. Conventional nodes cannot be split further without adding fiber and do not typically contain spare (unused) fibers, and thus there is a need to utilize the limited fiber availability in a more efficient and cost-effective manner.
Coherent technology has been proposed as one solution to increase both receiver sensitivity and overall capacity for WDM-PON optical access networks, in both brown and green field deployments. Coherent technology offers superior receiver sensitivity and extended power budget, and high frequency selectivity that provides closely-spaced dense or ultra-dense WDM without the need for narrow band optical filters. Moreover, a multi-dimensional recovered signal experienced by coherent technology provides additional benefits to compensate for linear transmission impairments such as chromatic dispersion (CD) and polarization-mode dispersion (PMD), and to efficiently utilize spectral resources to benefit future network upgrades through the use of multi-level advanced modulation formats. Long distance transmission using coherent technology, however, requires elaborate post-processing, including signal equalizations and carrier recovery, to adjust for impairments experienced along the transmission pathway, thereby presenting significant challenges by significantly increasing system complexity.
Coherent technology in long-haul optical systems typically requires significant use of high quality discrete photonic and electronic components, such as digital-to-analog converters (DAC), analog to digital converters (ADC), and digital signal processing (DSP) circuitry such as an application-specific integrated circuit (ASIC) utilizing complimentary metal-oxide semiconductor (CMOS) technology, to compensate for noise, frequency drift, and other factors affecting the transmitted channel signals over the long distance optical transmission. Coherent pluggable modules for metro solution have gone through C Form-factor pluggable (CFP) to CFP2 and future CFP4 via multi-source agreement (MSA) standardization to reduce their footprint, to lower costs, and also to lower power dissipation. However, these modules still require significant engineering complexity, expense, size, and power to operate, and therefore have not been practical to implement in access applications.
There could be many services that coexist in cable's optical access networks such as the traditional subcarrier multiplexed analog video, digital video and DOC SIS data services along with the less common radio frequency over glass (RFOG), EPON, Point-to-Point digital fiber links and others. When these are aggregated together, or even worse when they are aggregated over fiber with RFOG and analog, optical beat interference (OBI) becomes a significant problem. There is a need for a system that provides services that coexist in cable's optical access networks, meets a bandwidth demand, and decreases problems associated with OBI.